This invention relates to thermal radiation detection apparatus comprising a first high impedance thermal radiation detector element shunted by a low leakage device, said detector element being disposed on a mount and placed to receive radiation from a scene and thereby change in temperature and produce an output signal between two terminals on the element, one terminal of the detector element being connected to an output, and a reference low leakage device, one terminal of which is connected via a high gain negative feedback loop to the other terminal of the reference low leakage device and also to the other terminal of the detector element via a common line.
Such apparatus is disclosed in an article by A. A. Turnbull and M. E. Cooke entitled "High resolution 64-element pyroelectric linear array IR detector" and presented at the Fourth International Symposium on Optical and Optoelectronic Applied Science and Engineering from 30th Mar. to 3rd Apr. 1987 at The Hague, Netherlands, and subsequently published in Proceedings of SPIE, Vol. 807, pages 92 to 97.
This article describes thermal radiation detection apparatus comprising, in sequence, a radiation chopper, a linear array of pyroelectric detector elements, an integrated circuit containing a corresponding array of impedance converting source followers for the element outputs and a multiplexer for sampling the source follower outputs to provide a single channel signal for subsequent amplification, digitisation and digital signal processing.
The operation of the high gain negative feedback loop ensures that any d.c. offset in the output voltage at the one terminal of the reference low leakage device is reduced to zero. Consequently any d.c. offset in the output voltage at the output terminal of the detector element is substantially reduced to zero. The low leakage device establishes a d.c. connection for the negative feedback loop and its leakage is low enough at zero voltage not to degrade the performance of the detector element.
The use of a low leakage device in parallel with a pyroelectric element is described in British Patent Specification No. 1,580,403. This device is preferably a pair of diodes connected in parallel in opposite sense. The device provides a d.c. return path for the offset compensation loop as well as limiting the detector element output voltage in the event of large temperature changes.
In such apparatus there are two particular problems. First, compensation of some kind has to be provided for the large signals produced by the detector elements of the array due to temperature changes of the whole assembly. External termperature changes produce either rising or falling ramps of temperature of the elements, due to their finite thermal connection with the environment, which, in turn, produce output signals. The effect can mask wanted signals. For example in a typical equipment in which F/1 infrared optics are used to collect scene radiation which is then chopped at 40 Hz, a signal of 7.times.10.sup.-4 volts peak-to-peak per degree of black body scene temperature differences may be produced. A temperature ramp of only 0.017 degrees per second would produce signals of 5.times.10.sup.-2 volts, two orders larger than the signal voltage for one degree in the scene. Since typically it is desirable to be able to discriminate scene temperature differences of some 0.15 degrees centrigrade, the necessity for temperature compensation is all the more essential.
It should be noted that, following a large heat input to the array, there is a delay of about one thermal time constant (typically 60 milliseconds) before the temperature ramp settles to the same rate of temperature change for all elements.
A second problem derives from the offset voltages from each source follower. Typically, each detector has a MOSFET or a JFET source follower to provide a coupling between the very high impedance of the detector and the more usual impedance levels found in electronic circuitry. The source followers are sampled in sequence by the multiplexer, typically using MOSFET switches. The multiplexed signal is then amplified and fed to an analogue to digital converter which digitises the output of each detector as it is multiplexed. The multiplexing and digitising are synchronised with the operation of the radiation chopper blade in front of the array of elements which alternately exposes the array to scene radiation and to radiation from the chopper blade. The sampling of the multiplexer takes place at the end of each exposure to allow as much time as possible for the temperature of the elements to change. By subtracting the digitised outputs from each element obtained at the end of consecutive exposures to scene and blade radiation, any offset from the source follower associated with each element is cancelled leaving only the signal associated with the difference in scene and blade radiations.
The average offset of an array of integrated MOSFET source followers may vary from one diffusion batch to another by as much as a third of a volt and on any one particular array it will change by typically 100 mV over the desired temperature range. Individual offsets along the length of any one array may however typically vary by as little as .+-.15 mV around the average. As explained above, in the system both these average and individual offsets are removed by later processing. But in practice, if d.c. coupling is desired, these average offsets could occupy the bulk of the available dynamic range of the following amplifier and analogue to digital converter, thus severely restricting the overall resolution of the system. It will be seen that it is very desirable, if not essential, to remove as much as possible of the offset from the output of each source follower before amplification and analogue to digital conversion are carried out.
The article by Turnbull and Cooke cited above discloses solutions to both of these problems. The first problem is solved by arranging for each element to be connected in opposition to an associated equal element which is mounted adjacent to the element but which is shielded from the scene radiation. Any temperature ramps affect the two elements equally, the opposite connection of the elements providing cancellation of the temperature ramp generated signals. The second problem is solved by providing a reference opposed paralleled diode pair at either end of the linear array in addition to the diode pairs across the detector elements, each reference diode pair having a MOSFET source follower as nearly identical to the source followers of the elements of the array as possible. The solution relies on the offsets of the reference source followers being the same as those of the active element source followers. The two reference source follower outputs are averaged and applied to the negative input of a high gain differential d.c. amplifier whose positive input is at earth potential. The amplifier output is applied to the common connection of all active elements and reference diode pairs, the negative feedback connection ensuring that the reference average outputs are brought to zero, compensating their offsets and hence also compensating the offsets of the active channels to the extent that their offsets are the same as those of the reference channels.
However, continued work by the Applicants has revealed disadvantages of the above temperature compensation solution. One disadvantage is that twice as many pyroelectric elements are required for a given number of active channels. Also, the reference diode pairs alone may not constitute a close enough d.c. replica of the active elements in parallel with their diodes over the whole of the required operational temperature range.